Fermentation (College Board AP® Biology)

Alcohol & Lactate Fermentation

• Sometimes cells experience conditions with little or no oxygen

• There are several consequences when there is not enough oxygen available for respiration:

  • There is no final acceptor of electrons from the electron transport chain

  • The electron transport chain stops functioning

  • No more ATP is produced via oxidative phosphorylation

  • NADH and FADH₂ aren’t oxidized by an electron carrier

  • No oxidized NAD and FAD are available for dehydrogenation in the Krebs cycle

  • The Krebs cycle stops

• However, there is still a way for cells to produce some ATP in low oxygen conditions through fermentation (anaerobic respiration - 'without oxygen')

Fermentation pathways

• Some cells are able to oxidize the NADH produced during glycolysis so it can be used for further hydrogen transport

• This means that glycolysis can continue and small amounts of ATP are still produced

• Different cells use different pathways to achieve this

  • Yeast and microorganisms use alcohol (ethanol) fermentation

  • Other microorganisms and mammalian muscle cells use lactate fermentation

Alcohol Fermentation in Yeast

• In this pathway NADH transfers its hydrogens to ethanal to form ethanol (alcohol)

• In the first step of the pathway pyruvate is decarboxylated to ethanal

  • Producing CO₂

• Then ethanal is reduced to ethanol by the enzyme alcohol dehydrogenase

• Ethanal is the hydrogen acceptor

• Ethanol cannot be further metabolized; it is a waste product

Alcohol Fermentation Diagram

The pathway of ethanol fermentation

Lactic Acid Fermentation in Animal Cells

• In this pathway NADH transfers its hydrogens to pyruvate to form lactic acid (also called lactate)

• Pyruvate is reduced to lactate by enzyme lactate dehydrogenase

• Pyruvate is the hydrogen acceptor

• The final product lactate can be further metabolized when oxygen becomes available or stored as glycogen

Lactic Acid Fermentation Diagram

The pathway of lactate fermentation

Metabolization of Lactate

• After lactate is produced two things can happen:

1. It can be oxidized back to pyruvate which is then channelled into the Krebs cycle for ATP production

2. It can be converted into glycogen for storage in the liver

• The oxidation of lactate back to pyruvate needs extra oxygen

  • This extra oxygen is referred to as an oxygen debt

  • It explains why animals breathe deeper and faster after exercise

ATP Yield in Fermentation

• In cells there is a much greater energy yield from respiration in aerobic conditions than in fermentation (anaerobic conditions)

• In anaerobic respiration glucose is only partially oxidized meaning only some of its chemical potential energy is released and transferred to ATP

  • The only ATP producing reaction that continues is glycolysis (~2 ATP)

• Because there is no oxygen to act as the final electron acceptor, none of the reactions within the mitochondria can take place

  • The stages that take place inside the mitochondria produce much more ATP than glycolysis alone (~36 ATP)

Comparing Aerobic & Anaerobic Respiration Table